Shrek and Fiona in Human Form
The Happily Ever After Potion is the potion that Shrek, Donkey and Puss in Boots stole from the Fairy Godmother in her cottage to make Fiona happy with him. If one drinks the vial, their true love and themselves will turn into their most beautiful forms.
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Shrek drank the potion along with Donkey in order to win Fiona's heart for good. After they drank it, a big rain storm formed for the potion to locate Fiona so it would effect her too. By morning, they were a human and stallion respectively. The potion also turned Fiona human again. Puss soon reads that side effects included burning, itching, oozing, weeping and was not intended for heart patients and those with nervous disorders and that if a kiss was not taken by midnight, then the effects would wear off. However, if one kisses their true love, the potion's effects will be permanent.
After Shrek enjoys being a human he goes to see Fiona but as Shrek and Fiona go different ways, Fiona suspiciously thinks that Prince Charming is Shrek. Upon seeing the sight, Shrek is devastated and is shot down by Fairy Godmother for his attempts to change himself. He then lets his wife go, letting her be happy with Charming.
Drowning in sorrow at the Poison Apple, Shrek comes to term with losing Fiona-- willing to let the effects of the potion wear off. Right before he can go further in his depressed life, Shrek discovers a secret plan concocted by Fairy Godmother and Prince Charming. The two were going to take advantage of the potion, fooling Fiona into thinking Charming was Shrek. Disgusted at the plan, the ogre and his gang set off to fight back against the evil duo to save Fiona.
Shrek then breaks into the castle, eventually defeating Fairy Godmother. Afterwards, midnight strikes, giving Fiona the choice to stay human alongside her husband. She chooses to stay an ogre, like Shrek when she met him. The two undergo a transformation reverting back to their "beautiful forms", including Donkey. While sad, he is told by Shrek that "he still looks like a noble steed".